In typical axial output fibers, light imparts the fiber output face ideally at angles normal to the fiber axis. Fresnel reflections result from the change in refractive index at the glass/air interface as the light exits the glass and enters the air. Fresnel reflection intensity is proportional to difference between the refractive index of the glass and the surrounding medium and well as to the angle at which the light imparts the interface in refractive indices. Fresnel reflections are at a minimum for light imparting refractive index interfaces normal to the interface plane.
In multimode, large core fibers of the type used in laser energy delivery, the mean angle of the light imparting the axial output face is normal to the fiber axis but the vast majority of the light exiting the fiber does so at angles off of normal to the fiber end plane. Simple lateral output fibers are produced by polishing a bevel on the tip of an optical fiber at an angle off normal to the fiber axis. Light imparting the off-normal polished fiber tip is reflected according to Snell's Law, due to the refractive index interface between the fiber core glass and the surrounding medium, where the angles of incidence for the rays in the fiber meet the conditions for total internal reflection (TIR). Where the surrounding medium is air, the refractive index difference is relatively large so that the off-normal angle of the bevel polish may also be large while accommodating the worst-case light ray angle within the fiber (for fibers or relatively low numerical aperture or relatively low order mode fill). Where the surrounding medium is of a refractive index more closely matching the fiber, the maximum off-normal angle that provides for reflection of all rays within the fiber is reduced proportional to the reduction in refractive index difference. It follows that a bevel tipped silica core fiber (refractive index˜1.46), designed for use in air (refractive index 1.00) and providing maximum off axis output will emit at least some light axially. rather than laterally, when immersed in aqueous media (refractive index˜∫1.33). Normal contaminants in any use environment may also cause leakage if they come into contact with the reflective bevel tip of a lateral fiber. For this reason, most lateral fibers use a protective cap, positioned about the (angle polished) lateral tip, to exclude moisture and other interfering materials.
The output of surgical lateral fibers is extremely distorted with respect to the output of standard axial fibers. The physical construction of the optical fiber required for delivery of the high energy density is quite different from the construction of optical fiber that is used in communications. Communications fiber is composed of a small, germanium-doped silica core surrounded by a thick, pure silica cladding and the fiber mass is typically composed of less than one percent core material. High energy optical fiber is primarily core material (pure silica), surrounded by a relatively thin, fluorine-doped silica cladding where the core typically represents 70% or more of the fiber mass. The relative thickness of the cladding layer on communications-type fiber is functionally equivalent to an extrapolation of the art disclosed by Pon where the CCDR is roughly 12.5 instead of 1.4. Were such fiber suitable for lateral fiber applications in surgery, the light reflected off a suitable polished bevel tip would impart an approximately flat surface in that the arc of the fiber cylindrical wall inscribed by the beam path is but a small fraction of the fiber circumference such that minimal Fresnel reflection amplification, no Snell reflections and minimal cylindrical distortion would result. Of course, such a CCDR is impractical for the core diameters required in laser surgery; the fiber would be approximately a 7.5 mm diameter rigid rod rather than the required flexible conduit. For fiber constructions suitable for surgical applications, the small CCDRs required give rise to larger portions of the fiber circumference being illuminated by the reflected energy, giving rise to the amplification of Fresnel reflections, the introduction of significant Snell reflections and large cylindrical distortions of the output beam profile.
U.S. Pat. No. 5,562,657 (Griffin) discloses a lateral optical fiber for surgery and ordinance ignition that utilizes a pure silica sleeve, fused about fluorine-doped silica clad, silica core optical fiber (abbreviated “silica:silica” fiber in the industry) for the purpose of permitting laser-forming of the reflective bevel tip and shielding against melt distortion in subsequent fusion of the beveled and sleeved fiber to a protective silica cap.
U.S. Pat. No. 5,428,699 (Pon) discloses a high cladding-to-core diameter ratio (CCDR) fluorine-doped silica:silica fiber for lateral output surgery where the additional fluorine-doped cladding thickness reduces the reflections and distortions within the lateral output of the fiber upon which a reflective bevel tip has been formed. Pon does not anticipate fusion of the heavily clad fiber within a protective silica cap but disposes the bevel tip loosely within the cap.
U.S. Pat. No. 5,537,499 (Brekke) discloses a silica:silica optical fiber for lateral output surgery where the fiber cladding is directly fused to the protective silica cap just within the area of the light output.
Pon address the issues of unwanted Snell reflections, Fresnel reflections and cylindrical distortions within the output of lateral fibers. These distortions and reflections are primarily a result of light exiting the optical fiber through the sidewall rather than through a flat surface that is orthogonal to the mean axis of light propagation within the fiber, as in standard, axial output fibers. The magnitude of Fresnel reflections and cylindrical distortions is dependent upon the off-normal angles at which the light rays traverse the refractive index barrier upon exiting the fiber. By increasing the overall diameter of the fiber (beyond what is optically necessary for axial light propagation through the fiber), the angular arc portion of the fiber circumference through which the light passes in reduced, reducing angle dependent Fresnel reflections. Increasing the effective focal length of the cylindrical lens formed by the fiber sidewall also reduces cylindrical distortions, but most importantly the majority of the rays reflected by the bevel tip impart the fiber sidewall at angles close enough to normal to evade total internal reflection as governed by Snell's law.
In Griffin and Brekke, eliminating the air space between the fiber cladding and the protective cap entirely minimizes the refractive index difference within the light path, thereby minimizing all reflections and distortions. In Griffin, the fiber cladding is fused within a silica sleeve that is, in turn, fuse within the protective cap. In Brekke, the fiber cladding is directly fused to the protective cap. The effective diameter of the fiber at the output, in both approaches, becomes the cap diameter, which is typically a great deal larger than the original fiber cladding diameter. Further, in surgery, the fibers are used in aqueous media such that the final refractive index barrier traversed by the emitted light is from silica (1.46) to water (1.33). Where this minimal distortion is problematic, as is applications in lower refractive index media, Griffin disposes a flat surface on the output cap normal to the output axis.
Brekke is extremely similar to Griffin but in Brekke the fiber cladding itself is spot fused to the cap just on the fiber side surface where the light exits, eliminating most reflections and distortions. As a practical matter, the act of fusing a bare fiber into a relatively massive protective cap melts the reflective surface slightly, resulting in some distortions in the initial reflection. Pon avoids distorting the reflective bevel face at the expense of some reduction in Fresnel reflections and cylindrical distortions in that an air gap remains between the fiber cladding and the cap inner wall.
While the reduction of distortions in Brekke and Griffin are far superior to that afforded by Pon, there is an unrelated advantage provide by the art disclosed by Pon. Particularly in applications where energy densities within the fiber are especially high and where the light acting upon the target produces great amounts of heat, for example in pulse laser surgical applications, the fused strategies disclosed by Griffin and Brekke fail due to rapid thermal expansion and contraction. The fusion processes result in local stresses within the composite silica structures that are prone to fracture when exposed to extreme temperature differences. In short, the total energies that may safely be used within the art disclosed by Brekke and Griffin are limited due to these residual stresses. The art according to Pon is far more robust in extreme applications.
The art disclosed in Pon is practically limited by the minimal options available in fiber CCDR (as well as other dimensional design constraints) to a fiber cladding diameter that is 1.4-fold the core diameter. The fibers described in Pon as the preferred embodiment have 400 μm and 600 μm cores and, while non-standard, are drawn (produced) from standard preforms, though typically rare and expensive. A 1.6 CCDR fiber would perform better than the 1.4 CCDR fiber disclosed in Pon, but the costs of producing such fiber are incompatible with the needs of the surgical application because the 1.6 CCDR preforms required are non-standard and would be extremely costly to produce. The art in Pon is, in fact, limited by these economic considerations. 1.4 CCDR fiber costs more than twice as much to produce as the standard, 1.1 CCDR fibers disclosed in Griffin and Brekke and more than ten-fold more than the alternative disclosed herein. Further, use of larger CCDR fiber throughout a device limits the flexibility wherein flexibility is desirable.
In that the costly 1.4 CCDR fiber is only required within the relatively short lateral output portion of the surgical device (˜1 mm of an ˜3 m assembly), efforts have been made to splice short sections of 1.4 CCDR fiber to lower cost fibers. Unfortunately, where communications optical fiber is extremely precise in core and cladding dimensions (with a dimensional tolerance of less than 1% on relatively small overall diameters, typically 125 μm), the dimensions of the large core, multimode fiber used in surgical devices is less reproducible at typically 2% tolerance on the cladding diameter. Fusion splices in telecommunications-type fiber are fairly simple and routine, owing to the precise and accurate dimensions of the fiber. Further, given the standard variability in CCDR of 0.02 that is typical for power transmission-type fibers, coupled with the imprecision in maintaining overall fiber (cladding) dimensions, the core diameters may vary by as much as 4% between production lots of fiber in the diameters of interest. Mating cores that vary by as much as 25 μm between the low cost, 1.1 CCDR fiber and the costly 1.4 CCDR fiber introduces severe complications. The severe mismatch in glass diameters between the low cost fiber and the high CCDR fiber introduces additional difficulties in producing efficient fusion splices.
Where the laser wavelengths of interest are permitting, the use of polymer clad silica for carrying the energy to the lateral tip eliminates the variability of the CCDR presented by silica:silica fibers, but the core dimensional mismatch problem is reduced by approximately 50%: it is not eliminated. Where the low cost, trunk fiber under fills the receiving 1.4 CCDR fiber section, the dimensional mismatch problem is minimal in that all of the light exiting the trunk fiber enters the core of the short, lateral emission section. Where the inverse is the case, light exiting the larger trunk core will enter the 1.4 CCDR fiber (lateral emission section) cladding, rendering the Fresnel and Snell reflection reduction strategy ineffective for these cladding modes.
One strategy for eliminating the possibility of over-filling the receiving fiber core is to produce the 1.4 CCDR fibers at slightly larger than “normal” dimensions, rendering the minimum core diameter that is possible larger than the maximum trunk fiber core possible. Similarly, one could produce the lower cost trunk fiber to smaller than normal dimensions. This strategy preserves the reflection reduction strategy but introduces new complications in device design by increases the physical dimension mismatch issues and the overall size of the lateral emissions section by ˜4% or reducing the available core diameter for input of the laser energy. Beyond dimensional mismatch of the trunk and lateral emission section cores, precise physical alignment of the cores is also necessary and physical alignment of extremely different diameter materials is far more challenging that matching identical diameters.
The use of polymer clad silica fiber is attractive from cost perspectives. A solution for forming high CCDR, silica:silica fiber sections on low cost, silica:silica or polymer clad silica fiber, that evades dimensional mismatch problems, would be of considerable utility.